In this study an anaerobic-anoxic SBR (sequencing batch reactor) was used in order to investigate the possibility of phosphorus removal utilizing nitrate as an electron acceptor, instead of oxygen in biological phosphorus removal processes. The reactor was supplied with synthetic wastewater, in which acetic acid (HAc) and phosphate were added at concentrations of 400 mg-COD/l and 15 mg-P/l. A conventional anaerobic-aerobic SBR was also operated to compare with the anaerobic-anoxic SBR. The objectives of this research are to examine (i) feasibility and stability of the systems, (ii) kinetics and stoichiometry of phosphorus release and uptake. The anaerobic-anoxic SBR operation resulted in a stable phosphorus removal and accumulation of phosphorus removing bacteria using nitrate as an electron acceptor. Immediately after inoculation from a phosphorus removing plant (Renpho system) phosphorus uptake was observed, indicating that phosphorus removing bacteria which are able to utilize nitrate were already present in conventional phosphorus removing sludge. Comparison of stoichiometry and kinetics with the conventional anaerobic-aerobic SBR system shows a similar potential for phosphorus removal by denitrifying organisms. Therefore in the design of phosphorus removal processes one should not be afraid of nitrate, but use it.

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